This invention relates to a photosensitive member, such as a photosensitive member utilizing a silver halide light-sensitive coating as the photosensitive layer, having improved speed. More particularly, this invention relates to such a photosensitive member having pyroelectric and photovoltaic polyvinylidene fluoride (PVDF), e.g., poled PVDF, in close proximity to (e.g., adjacent) the photosensitive layer of the photosensitive member to increase the sensitivity of the photosensitive member.
It is known to use various kinds of synthetic resin for at least part of the support of the photosensitive layer of, e.g., a silver halide emulsion. Thus, U.S. Pat. No. 2,423,749 to Austin discloses a film base for a photographic element composed of an orientable vinyl fluoride polymer, the base bearing a stratum of a radiation-sensitive material such as a lightsensitive silver halide. Attention is also directed to U.S. Pat. No. 3,497,357 to Wang, which discloses a film base made from polyvinyl fluoride to receive photographic transfer images.
U.S. Pat. No. 4,087,804 to Stephens and U.S. Pat. No. 4,066,814 to Chiklis disclose the use of fluorinated polymers (including polyvinylidene-containing polymers) for anti-reflection layers on the back of silver halide film supports. The fluorinated polymers are useful for such layers because they have relatively high refractive indices.
The process by which latent images are formed in silver halide photosensitive coatings is as follows. When radiation quanta are absorbed by silver halide, electrons are ejected from the halide ions, leaving halogen atoms, and these electrons migrate randomly at thermal energies through the conduction band of the crystal. Some of the photoelectrons are eventually trapped at defect sites where silver atoms are formed, and small clusters of silver atoms of high stability formed in this way serve as the latent image for the standard photographic development process. The sensitivity or "speed" of silver halide-based products depends on, among other things, the efficiency of latent image formation.
There has been a steady effort to improve the sensitivity of silver halide-based products to provide a faster photosensitive product, because faster products have significant commercial advantages. For example, camera film having improved sensitivity allows photographing at shorter exposure, thus minimizing blurring from moving objects, and it allows fine grain photographs to be made at lower light levels. Faster X-ray film reduces the radiation hazard from some medical radiographic procedures. The commercial products in which improved sensitivity is desirable include silver halide systems used in photocopying, printing plates, and other graphic arts applications as well as in camera and X-ray products.
The efficiency of latent image formation, on which sensitivity depends in part, is limited by several factors. First, a part of the absorbed radiation produces only local heating, even when the emulsions contain color sensitizers and when intensifying devices such as phosphor screens are used. In addition, some of the photoelectrons that are produced recombine with the halogen atoms and make no net contribution to the formation of a latent image. If the net yield of photoelectrons were increased, e.g., recombination of halogen and photoelectrons were reduced, the efficiency of latent image formation would increase, and therefore the sensitivity of the silver-halide based product would increase.
When exposure is carried out with an electric field across the emulsion, the photoelectric current in the silver halide crystals is increased and recombination of halogen and photoelectrons is reduced. However, facilitating the formation of a stable latent image by imposing a voltage across the coating during exposure is cumbersome in practice.